1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED) structure, and more particularly, to an organic light emitting diode structure for avoiding over concentrated current and defects, such as cracks and spots.
2. Description of the Prior Art
In various types of flat panel displays, since an OLED, being developed later than a liquid crystal display (LCD), has many beneficial characteristics, such as having a spontaneous light source, a wide viewing angle, high response velocity, power saving, strong contrast, high brightness, small thickness, full-color, simpler structure, and a wide operating temperature, the OLED has been used extensively in small and medium scale portable display fields. After continuous research and development by manufacturers and scholars, the break through of some unresolved problems, such as low yield rate, unsatisfied mask application, unstable cap seal, has provided eminent progress. In the future, the OLED is even probably to be used in the large scaled display field.
Referring to FIG. 1. FIG. 1 is a cross-sectional view illustrating a conventional OLED structure 10. As shown in FIG. 1, the conventional OLED structure 10 mainly comprises a transparent glass substrate 12, a transparent conductive layer 14 being used as an anode of the OLED structure 10 and disposed on the glass substrate 12, an organic thin film 16 disposed on the transparent conductive layer 14, and a metal layer 18 being used as a cathode of the OLED structure 10 and disposed on the organic thin film 16.
The transparent conductive layer 14 comprises an indium tin oxide (ITO) layer or an indium zinc oxide (IZO) layer. The organic thin film 16 further comprises a hole transport layer (HTL) 20, an emitting layer (EL) 22, and an electron transport layer (ETL) 24 disposed on the transparent conductive layer 14 sequentially. The organic thin film 16 is formed by a thermal evaporation method. The hole transport layer 20 is composed of a diamine compound. The metal layer 18, composed of a metal having a low working function or an alloy, comprises a magnesium layer (Mg layer), an aluminum layer (Al layer), or a lithium/silver alloy layer (Li/Ag alloy layer).
In practical application, a hole injection layer (HIL, not shown) may be disposed between the transparent conductive layer 14 and the hole transport layer 20, and an electron injection layer (EIL, not shown) may be disposed between the metal layer 18 and the electron transport layer 24, depending on the requirements of devices and processes. Therefore, the problem of poor junctions of the organic thin film 16 and the anode/cathode is improved to help electrons or holes injected into the organic thin film 16. Furthermore, the emitting layer having an ability of transporting electrons and the hole transport layer having an emitting ability are optionally utilized to reduce the use of the organic thin film so as to simplify processing.
When a DC voltage is applied to the OLED structure 10, electrons in the metal layer 18 (cathode) and holes in the transparent conductive layer 14 (anode) will be injected into the emitting layer 22 through the electron transport layer 24 and the hole transport layer 20, respectively. Due to the potential difference incurred from the external electrical field, electrons and holes will move in the emitting layer 22 and recombine as excitions. When the excitions come back to the ground state by way of releasing energy, a specific percentage of energy (e.g. the quantum efficiency) is released in a form of photons to emit light downwards through the glass substrate 12. This is the electroluminescent principle of the OLED structure 10.
However, such a perfect situation must be based on the premise that each layer in the OLED structure 10 is undamaged. Referring to FIGS. 2–4. FIGS. 2–4 are cross-sectional views illustrating defects in a conventional OLED structure 50. As shown in FIG. 2, when forming a transparent conductive layer 54, an entire transparent conductive thin film (not shown) is formed first, followed by a wet etching process by utilizing an oxalic acid to form the transparent conductive layer 54. Since the wet etching process tends to incur the undercut phenomenon, and the crystallinity of the upper portion of the transparent conductive thin film (not shown) is superior to that of the lower portion of the transparent conductive thin film (not shown), the thin film in the lower portion of the transparent conductive thin film is more easily etched. As a result, an edge 58 of a top surface of the transparent conductive layer 54 becomes a right angle. In addition, the organic thin film 56 has a characteristic of poor step coverage ability to cause a very thin organic thin film 56 at the boundary 62 of the transparent conductive layer 54 and the organic thin film 56. When applying a voltage on the OLED structure 50, the current density at the boundary 62 of the transparent conductive layer 54 and the organic thin film 56 is too high, owing to electrical field distribution, causing rapid deterioration of the organic thin film 56.
As shown in FIG. 3, when the etching problem of the transparent conductive layer 54 becomes more severe, the edge 58 of the top surface of the transparent conductive layer 54 even becomes an acute angle. Thus, the deposited organic thin film 56 does not adhere to a sidewall of the transparent conductive layer 54 successfully, resulting in the crack phenomenon of the organic thin film 56 at the boundary 62 of the transparent conductive layer 54 and the organic thin film 56. As shown in FIG. 4, when the deposition or etching uniformity of the transparent conductive layer 54 is too bad in an extreme case, and a thickness of a portion of the transparent conductive layer 54 is greater than a thickness of the organic thin film, the transparent conductive layer 54, being used as the anode, may even cause a short circuit with a metal layer 64, being used as a cathode, at the boundary 62 to generate spots on products.
Therefore, it is very important to develop an OLED structure to avoid the over high current density problem incurred from the concentrated electrical field at the boundary of the transparent conductive layer and the organic thin film. Not only is the crack phenomenon of the organic thin film at the boundary of the transparent conductive layer and the organic thin film improved, but also the probability of generating spots in the OLED structure due to the short circuiting of the anode and the cathode is reduced.